The present invention relates to devices which couple to process control loops of the type used in industry. More specifically, the invention relates to validating a control signal from a field device.
Process control loops are used in industry to control operation of a process, such as an oil refinery. A transmitter is typically part of the loop and is located in the field to measure and transmit a process variable such as pressure, flow or temperature, for example, to control room equipment. A controller such as a valve controller is also part of the process control loop and controls position of a valve based upon a control signal received over the control loop or generated internally. Other controllers control electric motors or solenoids, for example. The control room equipment is also part of the process control loop such that an operator or computer in the control room is capable of monitoring the process based upon process variables received from transmitters in the field and responsively controlling the process by sending control signals to the appropriate control devices. Another process device, which may be part of a control loop, is a portable communicator which is capable of monitoring and transmitting process signals on the process control loop. These are often used to configure devices which form the loop.
It is desirable to validate the control signals in the process control system thereby improving the reliability of the entire loop. Typically, the prior art has been limited to simple validation techniques, such as monitoring a control signal and sounding an alarm or providing a safety shutdown if the control signal exceeds predefined limits. Another prior art technique is to generate a redundant control signal using the same or a different control algorithm and compare the two control signals. The control signal is invalidated if it differs from the redundant signal.